The standard frequency of the radio frequency generators mostly used in the industry today is 13.56 MHz. This frequency is open for industrial use by international telecommunication regulations. However, lower and higher frequencies were discussed and desired from the pioneering days of plasma capacitor applications. Nowadays, namely in PECVD applications, (plasma enhanced chemical vapor deposition applications), there is a trend to change to RF frequency values higher than 13.56 MHz, the preferred values being 27.12 MHz and 40.68 MHz (harmonics of 13.56 MHz). Higher frequencies allow for higher deposition rates in PECVD processes and thus increase productivity and lower product costs. Accordingly, this invention applies to RF frequencies in the range of 1 to 100 MHz, but it is mostly relevant to frequencies above 10 MHz. Furthermore, the invention can also be applied up to the microwave range of several GHz.
With large area plasma processing equipment, severe problems arise when the RF frequency is higher than 13.56 MHz and a large size (large surface) substrate is used. As described below, the problem addressed by this invention becomes of real importance when the largest dimension of the planar capacitive reactor (the diagonal) is equal or larger than 3-5% of the free space wavelength of the RF electric power driving the plasma. Under such circumstances, the reactor size is no longer negligible relative to the free space wavelength of the RF electromagnetic wave. In such a case, the plasma intensity along the reactor can no longer be uniform. Physically, the origin of such a limitation lies in the fact that the RF wave is distributed according to the beginning of a “standing wave” spatial oscillation within the reactor. Other non-uniformities can also occur in a reactor, for example non-uniformities induced by the reactive gas provided for the plasma process.
U.S. Pat. No. 6,228,438 of the same applicant (hereinafter US '438) describes different ways of solving the standing wave problem, which leads to voltage non-uniformity distribution over the reactor electrodes. U.S. Pat. No. 6,631,692 describes a plasma CVD film-forming device wherein both electrodes have a concave surface. According to U.S. Pat. No. 6,631,692, this leads to a more uniform plasma, but this document does not address the “standing wave problem” described below. US '438 focuses on the problem of an essentially cylindrically symmetric parallel plate reactor. No known prior art addresses the complex compensation necessary for the case of a square box shaped reactor with rectangular or square substrates and electrodes.
When a standing wave forms in the cavity, the voltage non-uniformity distribution can lead to plasma non-uniformity as the plasma is sustained in the reactor gap (between the cathode and the anode and above the substrate). This will lead to non-uniform processing and/or non-uniform properties of the layers on the substrate, depending on the desired application (deposition and etching for example). The present invention can also be applied to reactors, which do not necessarily use plasma: for example reactors using high frequency electromagnetic waves for heating.
To understand and to predict the standing wave problem, experiments have been conducted to determine the shape and the intensity of this non-uniformity and its dependency on the reactor scale (size) as well as the excitation frequency. US '438 teaches that the non-uniformity induced by standing waves depends very closely on the reactor size and the excitation frequency. Experiments were conducted in two types of reactors:    i) A large cylindrical reactor (with 1 m diameter) where non-uniformity due to standing waves is very pronounced at high frequencies (67.8 MHz and 100 MHz) was used for quantitative studies. FIG. 1-a shows the measured and normalized plasma light intensity for two extreme Argon plasma conditions (pressure and RF power) across the reactor diameter. The plasma conditions in this experiment have been chosen such that all other plasma conditions would have lead to a light intensity distribution, which is between the red and green curves. In the absence of plasma, the electrical field can be calculated to be zero in a distance of 2400 mm from the central RF injection point. By igniting plasmas under different power and pressure conditions at 67.8 MHz, it can bee seen in FIG. 1-a, that near zero plasma density already occurs at around 450 mm distance from the reactor center. This dramatic decrease in uniformity in presence of the plasma is due to the reduction of the electromagnetic wavelength causing the standing wave compared to the vacuum calculation and is due to the effective permittivity of the plasma and sheath distribution in the inter-electrode gap. By further increasing the plasma excitation frequency to 100 MHz, it can be seen that the zero plasma light intensity zone has shifted from 400 mm in FIG. 1-a to about 300 mm in FIG. 1-b.     ii) A small rectangular reactor (0.4 m×0.4 m) was used to measure the ion flux uniformity in Argon plasma. In FIGS. 2-a, 2-b, and 2-c, the plasma uniformity measured by ion flux uniformity in the plasma is shown. This ion flux uniformity is directly linked to electron and ion density uniformity. It should be noted that ion density and electron density uniformity of the plasma are, at a first order, the parameters, which are responsible for the plasma processing uniformity. From these figures it can be seen that the plasma is relatively uniform at 13.6 MHz and it becomes non-uniform when the excitation frequency is increased to 60 MHz and 81 MHz. This non-uniformity is due to the standing wave effect, which is more pronounced when excitation frequency is increased.This experimental evidence shows that the plasma non-uniformity, which is due to standing waves, is dependant on excitation frequency and reactor size. I other words it is dependant on the scaling difference between the excitation wavelength and reactor typical dimension.Known Problems for the Rectangular Case:
US '438 does not specifically refer to the standing wave problem in the case of a rectangular reactor and also in the case of very large reactor areas (>1 m2 and more typically 3-4 m2) where additional practical problems for RF injection points arise. When the reactor area increases, one needs to increase the number of injection points on the excited electrode (cathode) in order to distribute the RF current over several points and thus reduce the RF current density and thereby also reduce the failure risks due overheating and thermal impact such as melting, mechanical deformation, fatigue and others more.
For rectangular reactors, which are widely used in PECVD production—for such applications and equipment as TFT Displays, Plasma Displays, Solar Cells and others—the standing wave form which is created by electromagnetic propagation has a non-cylindrical symmetry shape (the solution still has symmetry along the two axes of the electrode) due to the strong effect of the reactor's corners or the electrode's corners respectively. In addition the waveform in the plasma region is dependant on the RF injection geometry, while this injection takes place in the backside of the electrode. Even if one uses only one RF injection point which is well positioned in the center of the back side of the reactor electrode, the effect of the corner due to the standing waves has to be taken into account. US '438 describes the shape of the compensating dielectric layer basically in cylindrical geometry with the thickness of the dielectric layer decreasing from the center towards the edges.
This method is strictly speaking optimized for cylindrical geometry reactors but cannot sufficiently compensate the non-uniformity when using a rectangular reactor.